The present invention relates to a vinyl chloride-based resin, referred to as a PVC hereinafter, composition or, more particularly, to a PVC composition capable of giving a colorless molded article emitting little offensive odor along with excellent mechanical properties, weatherability, heat stability, resistance against chemicals and transparency, especially, suitable as a molding material of containers of food.
Shaped articles of a PVC composition in general are inexpensive and have excellent mechanical properties, resistance against chemicals, weatherability, colorlessness and transparency and the flexibility or rigidity thereof can be freely controlled by compounding the PVC composition with a suitable amount of a plasticizer so that they are employed in a wide field of applications including not only household commodities but also a variety of industrial materials.
A defect in PVCs in general is that the resin is usually relatively poor in the heat resistance to cause discoloration at the temperature which the PVC composition encounters in the course of the molding process unless the PVC composition is admixed with a substantial amount of a stabilizer. Various types of stabilizers are known and under practical use in the prior art, of which metal-containing organic compounds are typical ones. When the PVC composition is used as a molding material of food containers, it is essential that the stabilizer is absolutely free from the problem of toxicity in addition to the requirement not to decrease the transparency of the articles molded from the PVC composition. Examples of stabilizers suitable for food containers are classified into three types including octyl tin mercaptide compounds, octyl tin maleates and combinations of calcium soap, zinc soap and an auxiliary agent, e.g., epoxidated soybean oil, organic phosphite compounds and the like.
These three types of stabilizers have their own advantages and disadvantages. For example, the tin mercaptide compounds exhibit excellent performance as a stabilizer in respect of the transparency, low initial coloring and heat stability of the shaped article of the PVC composition while they are not suitable for use in food containers because of the very unpleasant odor characteristic in mercaptans, although these compounds are most promixing for use in food containers if it were not for the malodor. The tin maleat compounds are free from the problem of malodor and capable of giving moderate heat stability to the PVC composition but are disadvantageous due to the relatively large initial coloring and yellowing of the shaped articles molded of the PVC composition containing the same. The metal soap-based stabilizers are defective due to the insufficient improvement in the heat stability of the PVC composition although they have no problems relative to the malodor and initial coloring of the shaped articles. Various attempts and proposals have been made to remedy the above mentioned defects in the conventional stabilizers, for example, by incorporating special substituent groups to the stabilizer compound or by the admixture of the PVC composition with a special additive but none of the attempts and proposals are far from complete solution of the problem.